In the manufacture of semiconductors, semiconductor devices are produced on thin disk-like substrates. Generally, each substrate contains a plurality of semiconductor devices. The exact number of semiconductor devices that can be produced on any single substrate depends both on the size of the substrate and the size of the semiconductor devices being produced thereon. However, semiconductor devices are becoming more and more miniaturized. As a result of this miniaturization, an increased number of semiconductor devices can be produced for any given area, thus, making the surface area of each substrate more and more valuable.
In producing semiconductor devices, substrates are subjected to a multitude of processing steps before a viable end product can be produced. These processing steps include: chemical-etching, wafer grinding, photoresist stripping, and masking. These steps typically occur in a process tank and often require that each substrate undergo many cycles of cleaning, rinsing, and drying during processing so that particles that may contaminate and cause devices to fail are removed from the substrates.
The importance of clean substrate surfaces in the fabrication of semiconductor devices has been recognized for a considerable period of time. The development of VLSI and ULSI silicon circuit technology has made the cleaning processes a particularly critical step in the fabrication process. Additionally, as the demand for semiconductor devices and the cost of labor increases, decreased production time has become a major concern in the semiconductor manufacturing industry. However, decreased production time must be achieved without compromising the quality of the resulting product or the yield of properly functioning devices per substrate.
In order to achieve these two goals, the use of acoustical energy (i.e., megasonic energy) during a number of processing steps has become common in the industry. The terms “megasonic energy” and “acoustical energy” are used interchangeably herein. Two processing steps in which the application of megasonic energy is particularly useful are cleaning and stripping. The application of megasonic energy during cleaning helps to more effectively remove particles from substrates while the use of megasonic energy during stripping can increase stripping rates.
In existing processes utilizing megasonic energy to clean substrates, a process tank is first filled with a cleaning solution such as standard clean 1 (SC-1), standard clean 2 (SC-2), deionized water, or a diluted variant of the aforementioned chemicals. SC-1 comprises 1 NH4OH: 1 H2O2: 5 H2O. SC-2 comprises 6 H2O: 1 H2O2: 1 HCl. Once the process tank is filled with the selected fluid and the substrates are submerged therein, a source of megasonic energy is coupled to the fluid for producing and directing sonic energy through the fluid and across the surfaces of the substrates. During megasonic cleaning, the transducer will oscillate at a megasonic rate between a negative and a positive position, generating negative and positive pressures within the fluid. As the megasonic energy oscillates, cavitation bubbles form in the fluid during negative pressure and collapse during positive pressure. Cleaning of the substrates come from two major occurrences: (1) cavitations (microscopic implosions); and (2) streaming (wave fronts that move the fluid along).
Similarly, megasonic energy can also be used to increase stripping rates during substrate manufacturing. During a megasonic stripping process, substrates are placed in a process tank in the presence of a fluid, such as liquid ozonated deionized (“DI”) water or a mist of ozonated DI water in an ozone gas atmosphere. Megasonic energy is then applied to the fluid as discussed above.
The standard means by which megasonic energy is produced is with piezoelectric crystals transducers. Piezoelectric crystals are pieces of ceramic which are metalized on both sides.
In order to transmit the megasonic energy to the fluid in which a substrate is being processed, existing systems use various types of rigid plates to connect the transducer to the tank. However, this set up has deficiencies with respect to the efficiency and effectiveness of the transmission of the megasonic energy from the transducers to the fluid. The transducers are directly bonded to one side of the rigid plate. Often, this bonding to the rigid plate is accomplished with less than ideal performance. The rigid plate is then coupled to a processing tank so that the side of the rigid plate that does not have the transducer affixed thereto is exposed to the processing fluid. One such prior art system is taught in U.S. Pat. No. 4,804,007, which is illustrated in FIG. 1. Referring to FIG. 1, piezoelectric crystals 5 are directly bonded to rigid plate 6. The piezoelectric crystals 5 are bound to the rigid plate by an epoxy bond. The rigid plate 6 is connected to a process tank so that the side of the rigid plate 6 that does not have the crystals 5 bonded thereto contacts the process fluid 7 in which the substrate is positioned. In utilizing this system, a high frequency energy source is applied across the piezoelectric crystals 5, causing the crystals 5 to create megasonic energy. This megasonic energy is transmitted from the crystals 5, though the rigid plate 6, and into the process fluid 7 within the process tank. It has been discovered that the megasonic energy that passes from the crystals 5 to the process fluid is significantly impeded due to the large differences in the acoustical impedance value (“Za”) of the consecutive materials (i.e., the crystals 5 and the rigid plate 6; and the rigid plate 6 and the process fluid). While epoxy is present to bond the crystals 5 to the rigid plate 6, the epoxy layer is so thin that its effect on acoustical impedance is negligible.
The Za of a material is defined as the product of the density of that material times the velocity of sound in that material. The units for Za are Mrayl or (kg/m2s×106). It has been discovered that the inefficient megasonic energy transmission of prior art systems is due in part to the differences in the Za of the materials through which the megasoinc energy must pass. More specifically, large differences in the Za between consecutive materials through which the megasonic energy must pass results in increased impedance of the megasonic energy and ineffective energy transfer to the process fluid.
The typical acoustical impedance of piezoelectric crystals that are typically used in a process tank utilizing megasonic energy transfer is Za=34 Mrayl while that of water is Za=1.5 Mrayl. Thus, in these systems, in order for megasonic energy to pass from the piezoelectric crystal into the water during substrate processing, the megasonic energy must undergo an acoustical impedance transition of approximately 32.5 Mrayl. While the rigid plates of prior art systems typically have a Za value that is between the Za of the crystals and the Za of the process fluid (e.g., quartz has a Za=12-15 Mrayl), the difference in the Za between the rigid plate and the fluid and/or the difference in the Za between the rigid plate and the crystals is still significant. This results in an undesired energy loss when the megasonic energy passes between the different materials. Additionally, prior art devices use the rigid plate only to connect the transducers (i.e., the crystals) to the tank and to protect the transducers from the fluid. The rigid plates are not intended or designed to smooth the difference in transition of acoustical impedance between the piezoelectric crystals and the process fluid.
Thus, in using systems, such as the one illustrated in FIG. 1, to process wafers, much of the megasonic energy that is created by the transducers 5 is not transmitted to the processing fluid 7 but is impeded from ever entering the fluid 7. Depending on the application, this energy loss can result in less than optimal cleaning and/or less than optimal stripping rates. Thus, existing systems that use megasonic energy to process substrates are less than optimal, resulting in energy transmission loss, increased energy usage, and/or less than optimal stripping and or cleaning performance.